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Identification of distortion sources in a power system is a topic unsolved. The problem has a difficult solution because there are elements in the system that do not produce harmonics, but amplify the existing ones in the electrical network. The most common of those elements is the capacitor, very used to compensate power factor at fundamental frequency. The capacitor behaviour makes the indices proposed up now to identify distortion indices fail in the presence of this element. In this paper, a new one is presented: the load characterization index. Besides using an extended equivalent circuit to represent the load, this index calculates the distortion introduced by the load, evaluating, besides the current distortion at its input, the voltage distortion at its terminals. The introduction of voltage assessing makes the index suitable to identify the linear and nonlinear loads in the power system, even in the presence of capacitors.